I’m not sure it’s that simple. When Jobs founded NeXT in 1985 there was no standard GUI layer for Unix. What you’re referring to as the “base layer” was all of

I’m not sure it’s that simple. When Jobs founded NeXT in 1985 there was no standard GUI layer for Unix. What you’re referring to as the “base layer” was all of Unix. X was only started in 1984 I believe and Sun also has a competing incompatible alternative at the time too. It wasn’t at all clear that the X Windowing System would win out as the dominant implementation.